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 Pulp
 Properties of paper
Properties of paper

Properties of pulp and the final product paper:

Strenghts
Tensile strength
Tensile strength is one of the key parameters in paper production. It is determined by mechanically placing a strip of paper into a tensile tester that strains the paper and registers the force needed to break it (measured in newton, N).

Lacking tensile strength may result from the cooking or bleaching process or from the type of wood used.
   
Tear strength
Tear strength describes the force required to propagate the tear of a paper with an initial cut through a specified distance. In order to define the parameter, the paper is cut and placed into a tear tester. The required force is expressed in millinewton (mN).
   
Burst strength
Burst strength is the pressure required to cause bursting of a paper. In order to define this parameter, a sheet of paper is placed in a bursting strength tester and subjected to an increasing pressure – generated by a rubber membrane – until it ruptures. The force required to burst the paper is called burst resistance and measured in kilopascal (kPa).

Fig.1: Tensile strength

Breaking length
Tensile strength and grammage form the basis for calculating the breaking length. It is the length of a paper strip in metres that would break due to its own weight if hanging freely.

 

Fig.2: Tear strength

Stretch at break
Stretch at break is measured together with tensile strength and expressed in percent. The percentage indicates how much the strip of paper has extended at the moment of break.

 

Fig.3: Burst strength

Plybond resistance
Plybond resistance describes the force needed to separate the layers of a paper, cardboard or paperboard (e.g. packaging paper, beer mats).

 

Opacity
Opacity refers to the degree to which a paper will transmit light, expressed in percent. A high degree of opacity prevents shining through of the print. The higher the opacity value, the more opaque the paper.


Smoothness

The rough surface of paper is evened out in a smoothing process. Irregularities have a negative effect on paper quality.

Yellowing
Yellowing of paper results from exposure to daylight and increased temperatures. The degree of yellowing is expressed in a yellowing index.

Fillers / Binders / Additives
Fillers improve printability, as they fill the gaps between the fibres. In paper production, natural mineral, inorganic substances such as calcium carbonate (chalk), kaolin (china clay), talcum, and also synthetically produced substances such as titan dioxide are used. Kaolin is also added to increase glossiness.

Fillers improve surface smoothness, calendering, opacity, brightness and softness. Calcium carbonate adds to the durability of a paper. Filler content may account for up to 30% of a paper's weight.

The fixing of the fillers is done using a retention agent and cationic starch won from maize or potatoes. The produced raw paper consists on average, considering all types of paper, to 4/5 of fibres and to 1/5 of natural fillers.

Optical brighteners and glues are considered additives. Glue regulates the absorbency of paper and is added to the paper mass to make the paper writable and to prevent the colours from blurring. Blurring of ink occurs in papers without glue, such as blotting paper for example.

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Brightness
Brightness is expressed in percent and measured using a brightness tester.

In the field of printing, the vibrancy of colour images depends on the brightness of the paper used. The higher the brightness of the paper, the higher the quality of the paper within its category. Optical brighteners may also be used.


Roughness
Roughness is expressed in micro metre (µm) and measured with a roughness tester. The closer the measured surface approaches the ideal surface (glass pane), the lower the measured value and the smoother the paper.

Residual lignin
The kappa number indicates the residual lignin in pulp. The lower the kappa number, the lower the lignin content. Low lignin content ensures greater age resistance of the paper.

pH-value
Age resistant papers consist of primary fibres (bleached pulp) and show a high pH value (approx. 8). For graphical papers standards have been defined with regard to age resistance, including limit values for the kappa number (<5). The pH value is set in the vat.

Specific volume
Voluminous papers are softer and show lower tensile strength, as they are thicker than others that have the same weight, and thus have a lower apparent density (<1.5). The volume is calculated based on the thickness and grammage.

 

Stiffness
Stiffness is an important benchmark of paper. It is the resistance to bending and its ability to support its own weight when handled. The degree of elasticity is mainly determined by the selection of the raw material and the refining of the pulp.

Porosity
Porosity depends on the type of pulp used, the refining of the raw material, the basis weight of the paper, as well as on the use of fillers. In general, every paper has air passages. Porosity expresses the extent to which a paper surface will allow the penetration of air and is measured with a porosity tester. The sheet of paper is placed into the tester which presses 100ml of air through a test area of 6.42cm² with 1.23 kPA and measures the time needed for this process.


Ash content
The ash content describes the residue left after complete combustion of paper and is expressed in percent. The higher the filler content, the higher the ash content of a paper.

Measurements are carried out according to international standards, which demand 50% humidity and 23°C room temperature prior and during testing.
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